The present invention relates to an optical disk apparatus, which records and reproduces information by use of laser beam, and to an optical head incorporated in the optical disk apparatus.
As a recording machine prevalent currently, a recording machine utilizing a videotape is in popular use. However, a recording machine utilizing an optical disk has been commercially available. The optical disk is more superior to the videotape in random access performance. From viewpoints of a easiness to handle, a repetitive reproduction, a fact that an image deterioration scarcely occurs due to change over time, compactness, or the like, it is considered that the recording machine using the optical disk will widely spread in the future. Furthermore, in addition to the recording, an optical disk apparatus is utilized for various kinds of applications such as an external recording device of a computer, a recording/reproducing apparatus of musical information, or the like. Therefore, it is considered that the optical disk apparatus gains increasingly in importance in the future.
In Japan, targeting at around Year 2003 to 2005, in a television broadcast, digitization of a satellite broadcasting, and digitization of a ground wave broadcasting are likely to be realized. Currently, a broadcast of higher definition moving pictures than a present analog broadcast will be widespread in ordinary homes, and thus it is considered that a demand for digitally recording this high definition moving picture will increase. In order to record this high definition moving picture for about 2 hours without impairing the quality of images, it is necessary that a large capacity storage of 20 to 25 GB in capacity is provided in a disk having a diameter of 12 cm which has the same size as a compact disk or DVD. In other words, it is necessary that the recording density is increased about 4 to 5 times the present DVD.
In order to increase the recording density, it is necessary that the laser beam source is shortened in wavelength, NA (numerical aperture) of the objective lens is increased, and a spot diameter for recording/reproducing information is reduced. At present, the wavelength of the laser beam source of DVD is about 660 nm, NA of the objective lens is about 0.6, and thus the recording capacity of 4.7 GB in one layer on single side is attained. On the other hand, as the laser beam source of short wavelength, a blue-violet semiconductor laser (wavelength 400 nm) is likely to be put into practical use. When this laser beam source is used, if NA is set to 0.85, 25 GB in one layer on single side can be accomplished.
As a prior art of increasing the NA of the objective lens, it is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-11402. Here, the NA is increased up to maximum 0.85 by use of the objective lenses of two elements. At this time, there is a problem that, the more the NA increases, the more an aberration generated due to a positional shift of an optical system, error of thickness and tilt of a disk substrate, or the like increases. In view of this problem, in the aforementioned prior art, in order to reduce a coma aberration generated due to the disk tilt, thickness of the substrate is thinned down to 0.1 mm. Moreover, as for a spherical aberration generated by error of thickness of the substrate, thickness of the substrate is detected from a difference between the focal shift signals of a surface of the disk and a record surface, and a position of a collimator lens is changed based thereon to compensate the spherical aberration.
In the aforementioned prior art, when the objective lens is offset from a central axis of the collimator lens in tracking operation in a state that the spherical aberration is corrected, the coma aberration occurs in a direction of shifting the objective lens. In the case where the spherical aberration due to the error of the substrate thicknesses is corrected, an amount of the coma aberration by this offset is small. However, in the case where the correction amount is relatively large as when the spherical aberration equivalent to an interlayer interval is corrected in recording and reproducing to and from two-layer disk, the coma aberration by this offset is affected largely, and a spot is deteriorated by the tracking operation, whereby the recording and reproducing of signals are adversely affected.
In order to reduce the influences of the coma aberration due to the offset of the objective lens, it is necessary to adopt a double servo system in which the optical head itself makes the tracking operation in conformity to revolution of the optical disk. However, since the optical head itself is moved in the double servo system, a mechanism system and a control system become complicated, and costs increase. Furthermore, since a movable part is large, this system is unfit for an increase in speed.